The present invention relates generally to the field of neuroprotectants and more specifically to the use of inhibitors of calcium-stimulated proteases, such as calpain, as therapeutics for neurodegeneration.
Neural tissues, including brain, are known to possess a large variety of proteases, including at least two calcium-stimulated proteases, termed Calpain I and Calpain II, which are activated by micromolar and millimolar Ca.sup.+2 concentrations, respectively. Calpains are a family of calcium activated thiol proteases that are present in many tissues. Calpain II is the predominant form, but Calpain I is found at synapses and is thought to be the form involved in long term potentiation and synaptic plasticity.
Thiol proteases are distinguished from serine proteases, metalloproteases and other proteases by their mechanism of action and by the amino acid residue (cysteine) that participates in substrate attack. Although several thiol proteases are produced by plants, these proteases are not common in mammals, with cathepsin B (a lysosomal enzyme), other cathepsins and the calpains being among the few representatives of this family that have been described in mammals. Calpain I and Calpain II are the best described of these, but several other members of the cysteine protease family have been reported.
Other Ca.sup.2+ activated thiol proteases may exist, such as those reported by Yoshihara et al. in J. Biol. Chem. 265:5809-5815 (1990). The term "Calpain" is used hereinafter to refer to any Ca.sup.+2 activated thiol proteases including the Yoshihara enzyme and Calpains I and II.
While Calpains degrade a wide variety of protein substrates, cytoskeletal proteins seem to be particularly susceptible to attack. In at least some cases, the products of the proteolytic digestion of these proteins by Calpain are distinctive and persistent over time. Since cytoskeletal proteins are major components of certain types of cells, this provides a simple method of detecting Calpain activity in cells and tissues. Specifically, the accumulation of the breakdown products ("BDP's") of spectrin, a cytoskeletal protein, has been associated with the activation of Calpain. In neural tissues, activation of Calpains, as evidenced by accumulation of these BDP's has been observed in many neurodegenerative conditions, including denervation resulting from focal electrolytic lesions, genetic abnormalities, excitotoxicity, Alzheimer's disease, following ischemia in gerbils and following administration of the toxins kainate and colchicine in rats, when administered peripherally or centrally.
Currently available Calpain inhibitors, which can cross the blood-brain barrier to inhibit Calpain in Central Nervous System (CNS) tissue, typically have limited solubility, thus making parenteral administration very difficult. Also may of these inhibitors are not calpain-specific and will inhibit a wide variety or proteases in addition to calpain.
Thus a need exists for calpain inhibitor compounds that possess good membrane permeability, are calpain-specific and have good solubility to permit parenteral administration.